Eco-hydrological Effects Of Different Vegetation Restoration Catchments In The Gully Region Of Loess Plateau

Anthropogenic activities and the land use change caused by them are the main reasons for accelerated soil erosion,which has a huge impact on regional and global terrestrial ecosystems.In the past thousand years,due to the rapid increase of population in the Loess Plateau,a large number of hillside fields had been cultivated and the vegetation had been damaged,which had greatly intensified the soil erosion from gully to slope in the Loess Plateau.The previous domestic research about soil erosion focused on the hilly and gully regions with highly developed loess hills and gullies in the Loess Plateau.However,with the effective control and restoration of the gully areas,the soil erosion of tableland areas has become the core and important issue in Loess Plateau.This study was carried out in two adjacent small catchments of the loess tableland area in Chinese Loess Plateau.The natural and artificial vegetation restoration models had been adopted in these two catchments,respectively.Various technologies and methods,such as unmanned aerial vehicle survey,hydrological observation,soil spatial analysis,topographical analysis,and erosion simulations,were combined to explore the differentiations of vegetation,soil,hydrology,and erosion in these two catchments.This study discussed the catchment eco-hydrological effects under the influences of ecology,hydrology,topography,and loess structure in the tableland areas of Loess Plateau.And the regulation mechanism of artificial afforestation on water and soil conservation was clarified at catchment scale.The study results could provide theoretical understanding of the soil erosion mechanism as well as the scientific and technological support for the soil erosion control in the gully region of tableland areas.The main conclusions are as follows:（1）It is found that the middle transition zone（slope>45°）is an important boundary separating the downhill and uphill slope in the artificial afforestation catchment.The distribution of the forest indexes（tree density,canopy height,canopy coverage,leaf area index）showed significant differences in the three topographic units.The canopy height,canopy coverage and leaf area index showed a downward trend from the gully bottom to the top of the catchment.The tree density showed a V-shaped change.Its low value area overlapped with the middle transition zone,and it was relatively high at the downhill and uphill slope.The hydrological environment was suppressed in the steep terrain area,which limited the vegetation density in the middle transition area.（2）The negative linear relationship between soil p H,soil total nitrogen and soil organic carbon indicated that the increased vegetation litter,soil total nitrogen,and organic carbon caused by afforestation may play an important role in the soil p H variations in the forestland catchment.The linear correlation between soil p H and soil total nitrogen and soil organic carbon in the downhill slope was more significant than that in the uphill slope,indicating that the soil total nitrogen accumulation and soil organic carbon from afforestation activities had caused the soil p H decrease in the downhill slope.In the forestland catchment,the soil p H was lower in the downhill slope than that in the uphill slope.In the grassland catchment,the soil p H was higher in the downhill slope than that in the uphill slope.The soil inorganic carbon was lower in the downhill slope than that in the uphill slope in both two catchments,which indicated that the strong physical and chemical weathering had occurred and the soil inorganic carbon had had been leached in the downhill slope.In the forestland catchment,the decrease of soil p H had accelerated the dissolution of soil inorganic carbon in the downhill slope,and the SIC was correspondingly low.In the grassland catchment,the frequent runoff events accelerated the dissolution and migration of soil inorganic carbon,especially in the rainy reason.（3）The soil particle size,water content,bulk density,porosity and saturated hydraulic conductivity were significantly different（p<0.01）in the grassland and forestland catchments.However,there was no significant difference in the above indicators between the middle transition zone,uphill slope and downhill slope in the grassland catchment;and there was significant difference between the three topographic units in the forestland catchment（p<0.05）.The middle transition zone was the high incidence area of water erosion and gravity erosion due to the steep terrain（slope>45°）.The vegetation cover of grassland catchment was dominated by grass,and it was weak for erosion resistance.The topsoil in the uphill and middle transition areas would be stripped and then transmitted downward due to water erosion and gravity erosion caused by heavy rainfall events.It means that the topsoil in the uphill slope could be denuded,transported and accumulated in the middle transition area,and the topsoil in the middle transition area would be denuded,transported and accumulated in the downhill slope.The vegetation was dense in the forestland catchment,and the complex tree and shrub roots firmly fixed the surface and deep soil,which could strongly block the surface runoff generated by rainfall events.Thus,it could reduce the surface soil erosion and migration,making a significant difference between the soil indicators of the uphill slope,the middle transition zone and the downhill slope.（4）The long-term afforestation almost completely obstructed the storm runoff occurrence in small catchments.The average runoff coefficient of naturally restored grassland catchment was ten times than that of the forestland catchment.Artificial afforestation vegetation had greatly affected the relationship between rainfall characteristics,antecedent soil moisture,and the storm runoff.The study found that there were thresholds for antecedent soil moisture and the sum of antecedent soil moisture index and event precipitation.Once the thresholds were exceeded,the storm runoff would significantly increase.However,no obvious threshold response relationship was found in the forestland catchment.We believe that the long-term afforestation had changed the mechanism of storm runoff in the Loess Plateau.And the rainfall intensity,precipitation amount,and the antecedent soil moisture jointly determined the hydrological connectivity between the uphill and downhill slope in the catchment.High density trees and understory vegetation play an important role in blocking the storm runoff process in the forestland catchment.（5）Afforestation is one of the main methods of vegetation restoration,and it plays an important role in water and soil conservation.The simulation results based on RUSLE showed that the average annual soil erosion modulus of grassland and forest watershed is 12.58 and 8.56 t ha^-1 a^-1 respectively,and the soil erosion modulus of forestland catchment was significantly lower than that of grassland catchment.High intensity soil erosion area(>80 t ha^-1 a^-1)was mainly distributed in the middle transition area（slope>45°）.In the forestland catchment,more than 90%of the high intensity erosion areas are distributed in the middle transition area.In the grassland watershed,more than 80%of the high intensity erosion areas are distributed in the middle transition area.The higher vegetation coverage has offset the negative impact of the higher annual precipitation in recent years,and it has effectively controlled the soil erosion,playing an important role in soil and water conservation in the Loess Plateau.